1. Field of the Invention
The present invention relates to a radar sensor for motor vehicles, having a transmit and receive component which includes a mixer for mixing a transmitted signal with a received signal; it also has an evaluation circuit which is connected to an output of the mixer by a direct voltage coupling device, as well as a compensation device for compensating a DC offset in the output signal of the mixer.
2. Description of the Related Art
Radar sensors in motor vehicles are used for tasks such as locating other vehicles within the scope of distance control systems or collision warning systems. Via an antenna, the transmit and receive component transmits a radar signal, which is modulated in its frequency in ramp-type manner. The radar echo received by the antenna is mixed with a portion of this transmitted signal in the mixer, so that a baseband signal is obtained whose frequency corresponds to the frequency difference between transmitted and received signal. Since this frequency difference is a function of the signal propagation time and the frequency shift caused by the Doppler effect, it provides information about the distance and the relative speed of the located object. The baseband signal is amplified and digitalized in the evaluation circuit and then forwarded to additional evaluation stages, e.g., a transformation stage, where the time-dependent baseband signal is broken down into its frequency spectrum by a fast Fourier transformation (FFT), in which the located objects manifest themselves in the form of individual peaks.
The transmit and receive component typically is formed by an MMIC (monolithic microwave integrated circuit), while the evaluation circuit typically is a user-specific integrated circuit (ASIC), a so-called RADAR-ASIC. The direct voltage coupling between the mixer output and the RADAR-ASIC makes it possible to analyze the entire information content of the baseband signal because no frequency components are lost by filtering out a direct voltage component. However, in the amplification of the baseband signal this type of coupling causes the direct voltage component contained therein to be amplified as well before the analog-digital conversion is able to take place. The DC offset amplified in this manner may cause the baseband signal to no longer fit into the voltage window of the analog-digital converter, so that overmodulation occurs, which leads to distortion errors in the subsequent Fourier transformation.
For this reason, known radar sensors include a compensation device, which compensates the direct voltage in the baseband signal before it is forwarded to the amplifier, the compensation taking place with the aid of current sources and resistors situated in the signal path. The current sources and the resistors for the compensation device are located in the RADAR-ASIC.
One disadvantage of this known design of the compensating device is that the resistors in the signal path required for the compensation contribute to an increased noise component and therefore have a detrimental effect on the quality of the signal, and thus on the quality of the radar detection. While it is in principle possible to suppress the noise by using lower resistance values and correspondingly higher current intensities, the current intensity is able to be increased only within certain limits, and it also contributes significantly to an increase in power losses. Until now the resistors must therefore be configured in such a way that a compromise is found between the contrary goals of low noise and low power losses.